Its previous owner haven't used it but it's pretty old. For example, instead of the current stamped logo as seen on the website, it's gold and says Mulberry (tree logo) Company, with Made In England below.

Anyhoo, my issues are:

1) The slots are rather tight for cards now. Is it OK if I wrestle 1/2 cards into them to stretch them out? I've been doing so for a month or so and it seems to be working, but I don't want to strain the seams or overwork the leather.

2) The cards get stuck to the slots and it's difficult to take them out, even when I've stretched the leather. Is there a treatment to make the leather less 'sticky'?

I've noticed the same tightness with any new canvas wallets, my husband have gucci and LV right now and both were stretched the way you did and i see no damage done to them. but they aren't sticky...you should look more into leather care :(